This invention relates to improvements in shadow mask color picture tubes and the manufacture thereof. Such a tube comprises a mosaic screen of sytematically arranged color phosphor elements, such as dots or lines, means including at least one electron gun for producing and directing a plurality of electron beams toward said screen, and a multi-apertured color selection shadow mask mounted between the electron gun means and the screen.
In a dot-type tube, the phosphor dots are usually laid down on the screen substrate, which is usually the tube faceplate, in trios or triangular groups of three circular dots of different color-emitting phosphors by a direct photographic printing technique wherein a photosensitive coating on the faceplate is exposed through circular apertures of the mask by a point source of light, and the coating is developed, as by washing off the unhardened portions, leaving the desired pattern of exposed hardened dots. This process is repeated for each color, e.g., red, green and blue. The shadow mask is preferably detachably mounted on the faceplate flange so that it can be easily removed and replaced in exactly the same position every time. Phosphor powder may, e.g., be mixed directly with each photosensitive coating before application to the faceplate, or else applied to the coating after the latter has been exposed.
In the operation of the tube, the electron beams are subjected to forces such as scanning (i.e., horizontal and vertical deflection) and dynamic convergence (to maintain convergence of the beams at the screen at various angles of deflection) which affect the electron beam paths and, hence, the landing points or spots on the screen, in ways that the screen-printing light rays are not affected. Thus, unless compensation is made for the differences between the beam paths and the light ray paths, serious misregister of the beam spots with the phosphor dots will result, i.e., the spot and dot centers will not coincide. The various kinds of misregister which may occur, and thus require compensation for, are described in columns 1 and 2 of Herzfeld et al. U.S. Pat. No. 3,476,025, issued Nov. 4, 1969. That patent discloses and claims a method of making a correction lens which, when used in the "lighthouse" for printing the mosaic screen of a shadow mask color tube, will refract the light rays from a point source in such manner as to provide acceptable compensation at each of a multiplicity of predetermined points distributed over the entire screen area for all conditions that tend to cause misregister of the spots and dots, at least for tubes up to 90.degree. deflection (the example given in column 6 of the patent was a 19 inches rectangular 90.degree. tube).
It has been found that, in making and using correction lenses for printing the screens of shadow mask color tubes, particularly those with substantially greater than 90.degree. deflection, e.g., 110.degree. deflection, undesirable conditions remain which are not susceptible to correction by a refracting lens. Such residual conditions include distortion of the triangles formed by the centers of the three dots of different colors in each trio away from equilateralism. The existence of such non-equilateral trios results in both overlapping of some dots of adjacent phosphor trios and unnecessary spacing of other adjacent dots. Depending on the kind of correction lens used in printing the mosaic screen, this non-equilateral distortion may be a maximum at the ends of the major and minor axes and a minimum at the center of the screen, or may be variable over the entire screen. In some cases, particularly where the screen is printed by the process of "second order printing". as disclosed in Morrel et al. U.S. Pat. No. 3,282,691, issued Nov. 1, 1966, the beam spots are substantially concentric or registered with their respective phosphor dots, even where the dot trios are distorted out of parallelism. However, in most cases, particularly at high deflection angles, the beam spots are exactly registered with the dots of the distorted trios. Depending on the amount of such misregister, this misregister may result in insufficient spot-dot tolerances, i.e. white uniformity or leaving tolerance, and/or color purity or clipping tolerance. White uniformity or leaving tlerance is the minimum distance (e.g. in mils) that a particular beam spot can move with respect to its respective phosphor dot without extending beyond (or beginning to leave) that dot. The leaving tolerance is, therefore, the shortest distance between the edges of the dot and its beam spot. Where the beam spots are smaller than the dots (positive tolerance tube) if a particular spot extends beyond its dot (negative tolerance dot condition) the amount of that particular color light emitted will be less than normal, which will change the white balance of the three color dots, and hence, reduce the white uniformity at that particular trio. The purity or clipping tolerance of a particular color dot is the shortest distance between the edge of that dot and the edge of the nearest beam spot associated with a different color dot, or the distance that such nearest beam spot can move toward the particular dot before touching or "clipping" that dot. Such clipping, in a tube used for color television picture reproduction, would result in a reduction in color purity at that point on the screen, because the clipping beam being modulated with color video information for one color would produce some light of a different color.